There is frequently a need to obtain DC power and constant-frequency AC power for DC and AC loads, respectively. For example, in aircraft and aerospace applications, variable-frequency AC power produced by a brushless, synchronous generator driven by an aircraft jet engine is converted by a rectifier bridge and a DC link filter into DC power on first and second DC link conductors. A static inverter coupled to the DC link conductors converts the DC power into constant-frequency AC power which is filtered and applied to frequency-sensitive AC loads. DC power for DC loads may be obtained directly from the rectifier bridge. The rectifier bridge and inverter together comprise a power converter known as a variable-speed, constant-frequency (VSCF) system.
The inverter used in VSCF systems may either be of the pulse-width modulated (PWM) type or the stepped-waveform type. An example of the latter type of inverter is disclosed in Compoly, et al., U.S. Pat. No. 3,775,662. The inverter is of the 24-step type and includes first through fourth inverter bridges coupled to first through fourth sets of three-phase primary windings of a summing transformer. The first and third sets of primary windings are connected in a wye configuration whereas the second and fourth sets of primary windings are connected in a delta configuration. A secondary winding is associated with each primary winding of the summing transformer and each secondary winding is connected in series with other secondary windings of the same phase to form three sets of series-connected windings. A summed voltage is developed across each set of series-connected secondary windings. The first and third inverter bridges are operated to produce identical waveforms in the first and third sets of primary windings except that the waveform produced by the third inverter bridge is displaced 15.degree. with respect to the waveform produced by the first inverter bridge. Likewise, the waveforms produced by the second and fourth inverter bridges are identical, except that the waveform of the latter lags the former by 15.degree.. In the event of an overload condition at the inverter output, the phase relationship of the currents from the inverter bridges is changed, so that the AC power from the summing transformer is reduced to a safe level. When the overload condition subsequently disappears, the original phase relationship of the currents is restored so that normal operation can continue.
An article entitled "Which DC/AC Inverter?" appearing in Electronic Design, Dec. 6, 1974, by George A. O'Sullivan discloses a 24-step waveform inverter similar to that disclosed in Compoly, et al. having four square-wave generators coupled to a summing transformer. This article indicates that output voltage control is possible utilizing a phase shifter which shifts two of the waveforms produced by two of the square wave generators with respect to the waveforms produced by the remaining generators. A feedback network is coupled between an output of the inverter and the phase shifter and a six-stage Johnson counter is coupled between the phase shifter and the square wave generators that develop the shifted waveforms.
Inokuchi, et al., U.S. Pat. No. 4,494,179 discloses a power conversion system including first and second power converters, one of which operates as a rectifier and the other which operates as an inverter together with regulation circuitry for regulating DC power flowing between the converters and a control for operating the inverter.